The present invention relates to a spray polymerization process, an apparatus for producing a dry polymer and a polymer having novel physical characteristics. Particularly, the present invention relates to a polymerization process for the continuous production in a controlled atmosphere of a substantially dry polymer particle powder comprising polymer particles of desired size, shape and density from a liquid monomer source.
It is known in the art that polymers may be synthesized by step polymerization and chain polymerization processes. Chain polymerization is initiated by a reactive species produced by a compound or compounds referred to as an initiator. Generally, monomers show varying degrees of selectivity with regard to the type of reactive center that will cause chain polymerization. Monomers show high selectivity between anionic and cationic initiators, however, most monomers will undergo polymerization with a radical initiator, although at varying rates. Examples of the types of monomers which will polymerize to high molecular weight polymers in the presence of a radical initiator include: ethylene; 1,3-dienes; styrene; halogenated olefins; vinyl esters; acrylates; methacrylates; acrylonitrile; methacrylonitrile; acrylamide; methacrylamide; N-vinyl carbazole; N-vinyl pyrrolidone.
Essentially, radical polymerization conditions are either homogenous or heterogeneous, depending upon whether the initial reaction mixture is homogenous or heterogeneous. Some homogeneous systems however, may become heterogeneous as polymerization proceeds due to the insolubility of the polymer in the reaction media. Generally, mass and solution polymerizations are homogeneous processes, while suspension and emulsion polymerizations are heterogeneous processes. All monomers can be polymerized by any of the various processes however, it is usually found that for commercial considerations the polymerization of a particular monomer is best carried out by one or two of the processes.
Bulk or mass polymerization of a pure monomer offers the simplest process with a minimum of contamination of the product. Bulk polymerization, however, is difficult to control due to the characteristics of radical chain polymerization. The bulk process is highly exothermic, high activation energies are involved, and there is a tendency toward the gel effect. Such characteristics make the dissipation of heat difficult, therefor, careful temperature control is required during bulk polymerization processes. Additionally, the viscosity of the reaction system increases rapidly at a relatively low conversion, thereby requiring the use of elaborate stirring equipment. Localized "hot spots" may occur which damage, degrade and discolor the polymer product, and a broadened molecular weight distribution may result due to chain transfer between polymer molecules. There is also the risk in extreme cases that an uncontrolled acceleration of the polymerization rate can lead to disastrous runaway-type reactions.
Many of the disadvantages of bulk polymerization may be overcome by polymerizing a monomer in a solvent (solution polymerization). The solvent, which may be water, acts as a diluent and aids in the transfer of the heat of polymerization. The solvent can be easily stirred since the viscosity of the reaction mixture is decreased. Although thermal control of a solution polymerization process is easier than with mass or bulk polymerization, the purity of the polymer may be affected if there are difficulties in removing the solvent during and following polymerization.
Heterogeneous polymerization is used extensively to control the thermal viscosity problems often associated with homogeneous processes. Precipitation polymerization is a heterogeneous polymerization process which begins as a homogeneous polymerization but converts to heterogeneous polymerization. A monomer either in bulk or in solution (usually aqueous but sometimes organic) forms an insoluble polymer in the reaction medium. Precipitation polymerization can be referred to as powder or granular polymerizations because of the forms in which the final polymer products are obtained. The initiators used in precipitation polymerization are soluble in the initial reaction medium and polymerization proceeds following absorption of monomer into the polymer particles.
Suspension polymerization, also referred to as bead or pearl polymerization, is carried out by suspending the monomer (discontinuous phase) as droplets (50 to 500 .mu.m in diameter) in water (continuous phase). The ratio of water to monomer typically will vary from about 1:1 to 4:1 in most polymerizations. The monomer droplets which are subsequently converted to polymer particles do not coalesce due to agitation and the presence of suspension stabilizers also referred to as dispersants or surfactants. Stabilizers may be. water soluble polymers or water insoluble inorganic powders. The suspension stabilizers are used typically in an amount that is less than 0.1 weight percent of the aqueous phase. The two-phase suspension system cannot be maintained in suspension polymerization without agitation.
Suspension polymerization initiators are soluble in the monomer droplets and are referred to as oil-soluble initiators. Suspension polymerization in the presence of high concentrations of water soluble stabilizers are used to produce latex-like dispersions of particles having small particle size. Such suspension polymerizations may be referred to as dispersion polymerizations. Inverse microsuspension polymerization involves an organic solvent as a continuous phase of a water soluble monomer either neat or dissolved in water. Inverse dispersion refers to systems involving the organic solvent as continuous phase with dissolved monomer initiator that yield insoluble polymer.
Emulsion polymerization involves the polymerization of monomers in the form of emulsions, i.e., colloidal dispersions. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. For most polymerization processes there is an inverse relationship between the polymerization rate and the polymer molecular weight. Large decreases in the molecular weight of a polymer can be made without altering the polymerization rate by using chain transfer agents. Large increases in molecular weight can be made only by decreasing the polymerization rate, by lowering the initiator concentration, or lowering the reaction temperature.
Emulsion polymerization allows increasing the polymer molecular weight without decreasing the polymerization rate. Emulsion polymerization has the advantage of being able to simultaneously obtain both high molecular weights and high reaction rates. The dispersing medium is usually water in which the various components are dispersed by means of an emulsifier. Other components include the monomer, a dispersing medium and a water soluble initiator. Surfactants are typically used in emulsion polymerizations at from 1 to 5% weight. The ratio of water to monomer is generally in the range 70/30 to 40/60 by weight.
The polymerization processes discussed above involve additional steps either to dry the polymer formed, separate the polymer from the organic solvent used in the process, or to recover the organic solvent. The added steps require additional energy and time in preparing the final product, thereby increasing the cost of the polymer produced. Moreover, the polymers produced using these known processes typically are produced as an agglomeration which must, following drying, be pulverized or in some way broken up to yield a usable polymer product. Breaking up the polymer product by grinding or pulverizing produces a substantial amount of dust which raises environmental and health concerns to those having to work in and around the polymer dust.
Therefore, there remains a need for a polymerization process and an apparatus in which to carry out the process which will enable the production of a dry polymer particle powder, thus eliminating the need to dry and pulverize the polymer product produced. There is also a need for a polymerization process for producing polymers which are immediately available for use following the completion of the polymerization process.
Finally, there is a need for a process to produce a polymer which allows the size, shape, and density of the polymer to be controlled easily and precisely. This is particularly important, for example, with polymers used in situations where a smooth surface is beneficial, such as in fiber optic cables. Fiber optic cables, which are becoming more common in telecommunications, are susceptible to invasive water. However, the use of super-absorbing polymers in fiber optic cables is problematical because of the relatively "soft" cladding around each fiber. The "softness" of the cladding makes it prone to scratching, which alters the refractive index of the cladding, and therefore, the ability of the fiber to conduct light. Experimentation has shown that superabsorbers produced by known processes cause very fine scratching of the cladding, an effect which is attributed to the rough surfaces of the polymer particles resulting from the pulverization, grinding, or chopping of the solid cross-linked polymer resulting from the above-described production methods into a fine powder as described above. The scratches in the surface of the cladding occur whenever the fiber optic cable is flexed, e.g., when the cable is wound on a spool and then wound off the spool for installation.
Polymerization processes frequently require that polymerization take place on a substrate or on a nucleating particle of some kind. For example, U.S. Pat. No. 4,135,043 discloses a process for manufacturing hydrophilic polymers. A previously formed polymer is coated with similar type monomers to form a coating on the polymer seed. Thereafter, the coated seed is heated in order to polymerize the coating thereon. Processes of this type also require that the additional production steps discussed above be employed.
Exemplary of the shortcomings of current polymerization processes are the known methods for the production of water-absorbing polymers. Such methods can be categorized as involving either an aqueous system or a multi-phase process. Aqueous systems for production of such polymers generally result in a semi-solid mass of material from which water must be removed in an energy-intensive drying step. For instance, in U.S. Pat. No. 4,295,987, the mixture of polymerized monomers must be dehydrated with excess methanol to form a firm solid that is dried in, for instance, a vacuum oven, and then ground into particles of a desired size or into a powder.
Also known are methods for continuous production of such polymers in an aqueous system as illustrated by the description set out in U.S. Pat. No. 4,525,527, hereby incorporated in its entirety by this specific reference thereto. Briefly, that patent describes the heating of an aqueous monomer solution to which an initiator is added by pouring the initiator onto the mixture as the mixture flows onto a traveling conveyer belt. The polymerization is exothermic, helping to drive off the water, and results in a "relatively dry, solid polymer of low water content", said to be 8-15% water. The solid polymer is then made into a powder by pulverization.
Multi-phase processes involve polymerization of an aqueous reaction mixture in an inert organic solvent, followed by the removal of the solvent from the polymerized product. So far as is known, such processes are batch processes, and a representative example of such a process is found in U.S. Pat. No. 4,446,261, hereby incorporated in its entirety by this specific reference thereto. That patent describes the preparation of polymer beads by suspension of an aqueous solution monomer and crosslinker in a hydrocarbon or halogenated aromatic hydrocarbon and polymerization of the monomer upon addition of a water soluble initiator. As described in that patent, the hydrocarbon is removed by distillation under reduced pressure and the residual polymer particles dried by heating under reduced pressure.
Other examples of such processes are found in the following U.S. patents:
AQUEOUS 3,661,815 3,669,103 4,071,650 4,167,464 4,286,082 4,295,987 4,342,858 4,351,922 4,389,513 4,401,795 4,525,527 4,552,938 4,612,250 4,618,631 4,654,393 4,703,067 MULTI-PHASE 4,059,552 4,093,776 4,340,706 4,446,261 4,459,396 4,666,975 AQUEOUS/MULTI-PHASE 4,062,817 4,654,039
Both types of processes are characterized by a number of disadvantages which add to the cost of producing such polymers such that there is a need for an improved method for producing these and other polymers. For instance, both aqueous and multi-phase batch processes require drying of the polymer.
Another disadvantage to producing polymers by known polymerization methods, particularly with respect to water-absorbing polymers, is the difficulty often experienced in controlling the size, shape, and density of the polymers produced. For example, water-absorbing polymer particles with a smooth external surface are, so far as is known by Applicants, are previously unknown. It appears that at least some multi-phase methods for production of such polymers result in spherical (see, for instance, column 5, lines 39, 48 and 60 of the above-incorporated U.S. Pat. No. 4,446,261) or donut-shaped (see column 6, line 57 of the above-incorporated U.S. Pat. No. 4,342,858) particles, but Applicants have been unable to find any such particles which have a smooth surface. Instead, all known particles are characterized by either a rough surface or by a surface such as that described in the above-listed U.S. Pat. No. 4,342,858 (column 6, lines 56-58) as being "high surface area donuts of collapsed spherical shapes with 2 to 5 micron protuberances on their surfaces".
Some additional disadvantages are characterized at, for instance, column 2, lines 56 et seq. of U.S. Pat. No. 4,093,776 and column 1, lines 18-56 of U.S. Pat. No. 4,625,001, both hereby incorporated in their entirety by this separate reference thereto.
It is, therefore, an object of the present invention to provide a novel polymer in which the size, shape and density of the polymer can be easily and precisely controlled, and a process and apparatus for doing so.
It is another object of the present invention to provide a novel polymer in which the degree of crosslinking and the water content of the resulting particle can be conveniently and precisely controlled, and a process and apparatus for doing so.
It is an object of the present invention to provide a novel polymer which is not formed on a substrate or other precursor which acts as a seed or nucleus for the polymer formation, and a process and apparatus for doing so.
It is an object of the present invention to provide a process and apparatus for producing a polymer which eliminates the difficulty inherent in the handling of the highly viscous solution, gel, or cake resulting from the production of such polymers with known processes.
It is another object of the present invention to eliminate the necessary grinding, pulverization, and/or chopping of a semi-solid mass of polymer which characterizes known processes for making polymers.
It is another object of the present invention to eliminate the costly step of recovering the organic solvent or medium used in known processes for the production of polymers.
Another object of the present invention is to provide a process and apparatus for producing a polymer which eliminates the costly drying step of many known processes for producing polymers.
It is an object of the present invention to provide a polymerization process which allows the polymerization reaction, polymer particle size, polymer particle shape, and water content of the polymer particle powder produced to be easily controlled.
It is an object of the present invention to provide a process and apparatus for using a fluid source of a selected monomer to produce a substantially dry polymer particle powder.
Other objects, and the advantages, of the present invention will be made clear to those skilled in the art from a review of the following detailed description of the presently preferred embodiments thereof.